Fewer THROUGH lanes move more traffic if the other movements are separately allowed for. I've seen cases where a 3-lane one-way street with parking is reduced to 2 wider lanes, 2 bike lanes, parking, and dedicated left/right turn lanes at major intersections. The traffic moves the same speed but more smoothly, because traffic isn't blocked in the outside lanes by turning movements and bicycles, and the hourly traffic flow is equal to or better than the 3-lane arrangement. The 3-lane street was effectively 1 through lane in the middle; now it has 2. Similar things happen if you have a 4-lane street with no center turn lane; the left lane is blocked by mid-block, and sometimes intersection, left turns. Dropping back to one lane each way with turn lanes and bike lanes improves traffic flow. Of course, these are all situations that occur in urban areas and business districts, not highways. I haven't seen "road diets" applied to highways, where it doesn't really make sense because the conflicting movements aren't there.

Yes, buses are pigs when not being used. So are trains. So is transit in general. That's why many transit systems have smaller buses for off-peak and neighborhood runs, most light rail systems have shorter trains off-peak, and almost all transit systems have longer headways off-peak for all but main-line routes. But to be a real public service there has to be something available all the time, so you have to accept less-than-full vehicles other than at commute times (and over-full commutes). That's one of the reasons why public transit will never make money and is a social service masquerading as transportation. In areas with really big differences between commute and off-peak, dial-a-ride in off hours has been used which is effectively a big taxi. And in some areas seniors can get "taxi scrip" that gives them discount taxi rides - effectively subsidized taxi service, which is more effective for very occasional use.

Again - public passenger transportation, however it's done, is a public/social service, not profit-making. For short periods, or on particularly favorable routes (think Megabus, NEC, non-subsidized Greyhound), some profit is possible, but not if you have to provide the whole network necessary for people to use it. ESPECIALLY if you have to cover the capital investment in the infrastructure - buses don't do that, since they share the road system that's already been paid for mostly by others, but rail lines do. If airlines had to cover the fully allocated cost of the ATC and airport systems, they would make losses much more often than they do now, and they would probably support in that case a return to regulation so high enough fares to cover that cost could be enforced.